Loss of plant yield due to plant disease is a global concern, not only in agriculture and horticulture but also in ornamental plants. Many valuable crop and ornamental plants are very susceptible to disease and would have difficulty surviving in nature without human intervention.
Cultivated plants are often more susceptible to disease than their wild relatives because large numbers of the same species or variety (which have a uniform genetic background), are grown closely together, sometimes over many thousands of square kilometres. A pathogen may spread rapidly under these conditions. For example, Phytophthora, a soil borne plant pathogen attacks the roots and stems of a range of plants, vegetable and fruits, is of particular concern to growers as it can contaminate water supplies and can also stay undetected in plant debris and soil for many years. It is estimated that Phytophthora, known as the “Plant Destroyer of the 21st Century”, alone causes a $2-7 billion loss per crop per year worldwide (Roy et al., 2012 Review of Plant Pathology, Vol 6).
Numerous methods exist to detect plant pathogens. Farmers typically use consultant agronomists who take a sample of soil or plant material, for example the leaf or root, and analyse the sample for plant pathogens. Analysis is conducted externally using laboratory tests. Such laboratory tests can include molecular techniques such as ELISA, PCR (PCR and real-time PCR), immunofluorescence (IF), flow cytometry, fluorescence in situ hybridization (FISH), and DNA microarrays. There are several problems with external laboratory testing of samples. Firstly, soil sampling selects only a small sample and may not necessarily reflect the true condition of the soil.
In some cases, the level of pathogen to be detected in the soil will be too low, therefore sampling an isolated area of soil the level of plant pathogen will be too low to be detected. External laboratory analysis requires transportation of samples away from the testing site to a laboratory and therefore there is a delay in providing the result of the diagnostic analysis. Any delay in detecting a plant pathogen can lead to further spread of the plant pathogen and a greater number of plants being affected.
Samples can also be tested for plant pathogens on site using lateral flow devices. Such devices require the farmer to take a sample from a plant, for example a leaf. The device extracts proteins in the plant sample and the presence of a plant pathogen can be detected. Each plant sample is representative only for the plant being tested. Each sample is therefore not representative of the entire plant growth area. Further, a plant sample that tests positive for a plant pathogen indicates the plant has already been affected by the pathogen. This may be too late to prevent spread of the plant pathogen to surrounding plants.
In some instances, the farmer may not detect plant pathogens at all and simply utilises preventative spraying routines against common plant pathogens. It is not known if the plants will be targeted by a plant pathogen if left untreated and therefore such spraying routines may be unnecessary in some cases and an unnecessary cost.
Early detection of plant health and disease could facilitate the control of disease through proper management strategies, such as vector control through pesticide applications, fungicide applications and disease-specific chemical applications. There is a need to provide an accurate and simple method or device for detecting a plant pathogen that can be utilised at the site of plant growth.